As to the concept of costume changes...
I am thoroughly convinced that the other generations' trainers will not be appearing. Why? Well, here's why:
As we all know, the purpose of a costume change is, first and foremost, so that players can tell themselves apart when one or more player is using the same character, and secondly to accomodate for the different team colors, which are red, blue, and green (not putting down the fact that one or two MAY be added). But those of you who have played the Pokemon games or seen the trainers know that every single male trainer, except for Brandon (third gen, dressed in black), is dressed in a red jacket/shirt, jeans, has dark hair, and a red hat. So, if we wanted to add in all of the trainers, we'd need...
13 models total. But here's an even bigger reason why multiple trainers will not appear:
Naturally, it wouldn't make sense for all of the trainers to use Squirtle, Ivysaur, and Charizard. No, they would have to use the correct starters for their region.
So, that means we'd have:
Squirtle, Ivysaur, Charizard
Totodile, Bayleef, Typhlosion
Mudkip, Grovyle, Blaziken
Piplup, Grotle, Infernape.
Each in three different colors.
Not only would we need so many models in so many different colors, we would also need seperate movesets for certain Pokemon. For example, each of the water starters could use water gun or hydro pump, but have very few attacks in common otherwise. For Ivysaur, Bayleef, and Grotle, an easy moveset would include tackle, razor leaf, and solarbeam, for example. But Grovyle shares none of those attacks. The same holds true for the fire starters, unless Sakurai wanted to have some horribly generic moveset, like Focus Punch, Flamethrower, and Earthquake. But I think many of us are expecting Charizard to Fly somehow, either as his up B attack, or naturally, and the other starters obviously can't do that.
So, no, the other trainers and starters will not be appearing in Smash Bros. as playable characters. As you can see, it's entirely too much work for the Sora Team, which has a lot more to worry about than Pokemon Trainers.